In producing rolled sheet metal products, it is often desirable to apply to a portion of the product a logo or pattern, to identify a particular brand, distinguish the product from other similar products, or to modify certain properties of the surface. The scale of surface modification need only involve microscopic changes in topography, or depth, relative to the sheet metal thickness.
In applying a textured pattern, or any discrete feature, it is highly desirable that the bulk elastic yield strength of the sheet metal not be surpassed. Exceeding the bulk elastic yield strength can cause modifications in the thickness, length or shape of the sheet metal. These modifications can affect the flatness of the sheet, resulting in wrinkling or buckling with consequent problems in handling and quality. However, in order to ensure that the desired patterning feature is distinguishable and clearly imprinted, it is necessary to apply sufficient pressure to modify the surface topography of the sheet material in the area of the desired feature.
Traditional patterning methods have used large uniform loads across the surface of the sheet metal, resulting in plastic deformation throughout the sheet as the pattern or logo is applied. As mentioned above, this results in reduction of the thickness of the strip (i.e. exceeding the through thickness yield strength of the sheet metal). Even very small reductions in metal thickness can cause a loss of flatness in the rolled product. Hence, conventional roll texturing of sheet surfaces is usually confined to rolling operations incorporating sophisticated methods of flatness control for the strip product.
A further problem in accommodating metal reduction as part of a rolling process to convert surface topography is pattern distortion. By definition, if a sheet article is made thinner by the applied forces in the roll bite, the sheet will extend in length, along the rolling direction. This elongation inevitably distorts any patterning features transferred to the strip from an embossing roll.
Prior art methods of applying microscopic patterns, commonly in the form of diffraction gratings or holographic images, invariably require relatively malleable sheet materials (e.g. metalized polymer films). This, in turn, requires that the sheet be heated prior to embossing. Heat is required since, in applying a typical diffraction grating or hologram at low temperature to a metal surface, the following dilemma is faced: a pressure sufficient to reproduce all of the detail can cause distortion of the metal substrate; whereas lower pressure, below the yield strength, can result in clarity and intensity of the pattern being lost.
Attempts have been made in the past to apply distinct patterns or logos to rolled sheet metal. European Patent Publication EP 338 378, herein incorporated by reference, discloses a technique and apparatus for transferring a hologram on a limited area of an end product by the use of raised portions on a roll, which contain the hologram pattern to be applied. PCT patent publication WO 02/072290, herein incorporated by reference, discloses a multi-roll rolling process for producing randomly textured sheet under conditions in which the rolling pressure is always less than the yield strength of the metal substrate.
A number of patent documents, such as Japanese Patent Publications JP 5104102 and JP 9085306, both of which are herein incorporated by reference, disclose forming roll surfaces with projecting and recessed protrusions to roughen sheet metal surfaces, however there is no teaching of achieving distinctive logos on the imprinted sheets.
Other patent documents, such as Japanese Patent Publication JP 59178144, herein incorporated by reference, disclose a method of producing an embossed plate of reduced weight by rolling the plate between two rollers having shoulder parts of recessing and projecting grooves so that the radius of curvature and the overall thickness of the embossed plate differ from the original plate.
Still further documents, such as U.S. Pat. No. 5,552,235, herein incorporated by reference, teach embossing methods in which one of the rolls is textured and the other is flat, but again, there is no teaching of creating clear and distinguishable logos, or of preventing changes in the sheet thickness.
U.S. Pat. No. 5,799,525, herein incorporated by reference, discloses means for embossing a cylindrical side wall of a can body by use of male and female tooling members having protuberances and indentations of various elevations to apply embossed features to the can body.
There is a need therefore for an improved process and apparatus for imprinting sheet metal strip with distinct logos or patterns, which can be used without the need to heat the sheet metal. There is also a need for a simple and effective means of applying microscopic patterns and logos to sheet metal without distorting the characteristics of the sheet metal or of the logos to be imprinted.